Preforms, composite components formed therewith, and processes therefor

ABSTRACT

A three-dimensional preform, composite components formed with the preform, and processes for producing the preform and composite materials. The three-dimensional preform includes first and second sets of tows containing filaments. Each tow of the first set has a predetermined polygonal cross-sectional shape and is embedded within a temporary matrix. The preform is fabricated from the first and second sets of tows, in which the second set of tows are transverse to the first set of tows, adjacent tows of the second set are spaced apart to define interstitial regions therebetween, and the polygonal cross-sectional shapes of the first set of tows are substantially congruent to the cross-sectional shapes of the interstitial regions so as to substantially fill the interstitial regions.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of U.S. patent application Ser.No. 11/566,370, filed Dec. 4, 2006, now U.S. Pat. No. 7,837,914. Thecontents of this prior application are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Agreement No.F33615-98-C-2893 awarded by the U.S. Department of the Air Force. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention generally relates to composite materials and theirrelated processes. More particularly, this invention is directed toprocesses of forming a three-dimensional textile preform whose structurecan be more readily densified and uniformly infiltrated to yield a densecomposite component.

Ceramic matrix composite (CMC) materials generally comprise a ceramicfiber reinforcement material embedded in a ceramic matrix material. Thereinforcement material serves as the load-bearing constituent of the CMCin the event of a matrix crack, while the ceramic matrix protects thereinforcement material, maintains the orientation of its fibers, andserves to dissipate loads to the reinforcement material. Of particularinterest to high-temperature applications are silicon-based composites,such as silicon carbide (SiC) as the matrix and/or reinforcementmaterial. As examples, SiC fibers (filaments) and tows (bundles offilaments) have been used as a reinforcement material for a variety ofceramic matrix materials, including SiC, TiC, Si₃N₄, and Al₂O₃.

Continuous fiber reinforced ceramic composite (CFCC) materials are atype of CMC that offers light weight, high strength, and high stiffnessfor a variety of high temperature load-bearing applications. A CFCCmaterial is generally characterized by continuous fibers that may bearranged to form a unidirectional array of fibers, or bundled in towsthat are arranged to form a unidirectional array of tows, or bundled intows that are woven to form a two-dimensional fabric or woven, braided,etc., to form a three-dimensional fabric. Conventional textile patternscan be used to form a textile preform in which two or more sets of towsare interlaced. The terms “warp,” “weft,” and “bias” are commonly usedto identify the orientation of tows relative to weaving processes, andthe terms “axial” and “braider” are commonly used to identify theorientation of tows relative to braiding processes. Warp and axial towsare those that, during the fabrication of a preform, continuously passthrough a weaving or braiding machine so as to be parallel to theprocess direction of the preform. Weft (or fill) and bias tows runtransverse (perpendicular and oblique, respectively) to warp tows of awoven preform, and braider tows run transverse to the axial tows of abraided preform. Because weft, bias, and braider tows are interwovenwith the warp and axial tows, the former group may be termed dynamictows and the latter static or stationary tows in reference to theweaving and braiding processes. Because the dynamic tows are interlacedwith the static tows, the static tows tend to be substantially straight.The individual tows may be coated with a debond interface, such as boronnitride (BN) or carbon, forming a weak interface coating that allows fordebonding and matrix crack deflection between the tows and the ceramicmatrix material. As cracks develop in the CMC, one or more fibersbridging the crack act to redistribute the load to adjacent fibers andregions of the matrix material, thus inhibiting or at least slowingfurther propagation of the crack.

CMC components having complex shapes and those subject to highmechanical and thermal loads typically require a tailoredthree-dimensional fiber preform architecture that is densified with aceramic matrix material, such as by infiltrating the preform with thedesired matrix material (or a precursor thereof) to fill the porositywithin the preform. Conventional three-dimensional preform fabricationprocesses (such as braiding, weaving, etc.) utilize dry or lightly sizedtow whose brittle ceramic filaments can suffer damage from thefabrication process. For those CMC components requiring preforms withlarge tow size (high filament counts) to obtain desired part dimensions,three-dimensional preform fabrication processes do not allow for directcontrol of “filament packing” within the tows or shaping of the towcross-sections to obtain optimized fiber weighting. The requirement oruse of large tow sizes, in conjunction with the abrasive nature of thetextile preforming process, can lead to filament breakage during preformfabrication and difficulties in matrix infiltration during compositedensification, both of which can negatively affect the mechanical andphysical properties of the CMC. To compensate for this, state of the artpreform fabrication methods often employ both a lightly sized tow toreduce filament breakage during preforming, and an arbitrarily set limiton size of tow or filament count to aid in proper matrix infiltration.Prior CMC work has indicated that a key to obtaining good fiber coatingand composite properties is related to spreading of the fibers insidethe tows of two-dimensional fabrics used to form a CMC preform. Fortwo-dimensional fabrics, this has been achieved by using mechanical andultrasonic fluffing techniques. However, such techniques are noteffective for three-dimensional fabrics.

In view of the above, it would be desirable if a process were availableby which tow shape and filament packing within tows can be more readilypredetermined and controlled while also providing for protection of thetow filaments during the preform fabrication process.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a preform, composite components formedwith the preform, and processes capable of producing the preform and thecomposite materials, particularly CMC components, to have desirableproperties as a result of enabling improved control over tow shape andtow filament packing and minimizing damage to the tows and tow filamentsduring the preform fabrication process.

According to a first aspect of the invention, a preform is produced by aprocess that includes producing first and second sets of tows, in whicheach tow contains a plurality of substantially parallel filaments. Inaddition, each tow of the first set has a predetermined polygonalcross-sectional shape and is embedded within a temporary matrix. Thethree-dimensional preform is then fabricated to comprise the first andsecond sets of tows, in which the tows of the second set are transverseto the tows of the first set, adjacent tows of the second set are spacedapart to define interstices therebetween having a polygonalcross-sectional shape, and the predetermined polygonal cross-sectionalshapes of the tows of the first set are substantially congruent to thepolygonal cross-sectional shapes of the interstices so as tosubstantially fill the interstices.

According to a second aspect of the invention, a preform comprises atleast first and second set of tows. Each tow of the first set of towscontains a plurality of substantially parallel filaments, has apredetermined polygonal cross-sectional shape, and is embedded within atemporary matrix. Each tow of the second set of tows contains aplurality of substantially parallel filaments. The tows of the secondset are transverse to the tows of the first set, adjacent tows of thesecond set are spaced apart to define interstitial regions therebetweenhaving a polygonal cross-sectional shape, and the predeterminedpolygonal cross-sectional shapes of the tows of the first set aresubstantially congruent to the polygonal cross-sectional shapes of theinterstitial regions so as to substantially fill the interstitialregions.

According to a third aspect of the invention, a gas turbine enginecomponent comprising a matrix material reinforced with athree-dimensional preform. The three-dimensional preform includes atleast first and second sets of tows. Each tow of the first set of towscontains a plurality of substantially parallel filaments and has apredetermined polygonal cross-sectional shape. Each tow of the secondset of tows contains a plurality of substantially parallel filaments.The tows of the second set are transverse to the tows of the first set,adjacent tows of the second set are spaced apart to define interstitialregions therebetween having a polygonal cross-sectional shape, thepredetermined polygonal cross-sectional shapes of the tows of the firstset are substantially congruent to the polygonal cross-sectional shapesof the interstitial regions so as to define substantially uniformspacing between the first and second sets of tows, and the matrixmaterial is within the spacing between the first and second sets of towsso that the preform is within the matrix material.

In view of the above, the temporary matrix on the first set of tows isable to provide improved control over the cross-sectional shapes of thetows, improve the filament packing within these tows, and minimizedamage to the tows and tow filaments during the preform fabricationprocess, without negatively impacting the properties of the finalcomposite component. According to a preferred aspect of the invention,the preform constructed and processed as described above can be heatedto remove the temporary matrix on the first set of tows so as to yieldsubstantially uniform spacing between the first and second sets of tows.Thereafter, the spacing between the first and second sets of tows can beinfiltrated with a desired matrix material or a precursor of the matrixmaterial to form an infiltrated preform body, and the infiltratedpreform body heated to yield the composite component. According toanother preferred aspect of the invention, the temporary matrix burnsoff during heating of the preform without leaving any amount of residueon the tows of the first set that would reduce the mechanical propertiesof the composite component.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a fragmentary cross-sectional view of athree-dimensional fabric preform formed by transverse sets of tows inaccordance with the prior art, and illustrates extensive anduncontrolled packing of filaments within static tows and inefficientfilling of interstices between adjacent dynamic tows.

FIG. 2 schematically represents a fragmentary cross-sectional view of afabric preform similar to that of FIG. 1, but in which the static towsare preshaped to have controlled filament packing, are embedded in aprotective temporary matrix material, and have cross-sections optimizedto fill interstices between adjacent dynamic tows in accordance with thepresent invention.

FIG. 3 schematically represents two different cross-sectional shapes fordynamic tows shown in FIG. 2.

FIG. 4 schematically represents the preform of FIG. 2 infiltrated with amatrix material to form a ceramic matrix composite component.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally applicable to composite materials,and particularly CMC materials suitable for components such as combustorcomponents, high pressure turbine vanes, and other hot section airfoilsand components of gas turbine engines. The invention also hasapplication to other components, including advanced power generationsteam turbines and various other equipment that can make use of CMCmaterials. Examples of CMC materials to which the invention pertainsinclude those with a silicon carbide, silicon nitride, and/or siliconreinforcement material in a ceramic matrix of silicon carbide, siliconnitride and/or silicon, e.g., a SiC/SiC CMC, though the invention alsoapplies to other types and combinations of CMC materials.

FIG. 1 schematically represents a fragmentary cross-sectional view of athree-dimensional fabric preform 10 of the prior art. The preform 10comprises two transverse sets 12 and 14 of tows 16 and 18, respectively.The first set 12 of tows 16 is represented as being static tows, forexample, the tows 16 are axial tows if the preform 10 has a braidedarchitecture or warp tows if the preform 10 has a woven architecture. Assuch, the transverse tows 18 of the second set 14 are dynamic tows, forexample, braided tows if the preform 10 has a braided architecture, orweft (fill) or bias tows if the preform 10 has a woven architecture. Asconventional, the tows 16 and 18 are bundles made up of numerousparallel fibers or filaments (not shown), which as noted above may beformed of silicon carbide, silicon nitride, silicon, and/or anotherceramic material. A notable commercial example is HI-NICALON® fromNippon Carbon Co., Ltd. Typical ranges for the diameters of thefilaments are about eight to about twenty micrometers, though fiberswith larger and smaller diameters are also possible. Typical fibercounts for the tows 16 and 18 are generally in the range of about 400 toabout 800, and typical ranges for the diameters of the tows 16 and 18are about 500 to about 2000 micrometers. However, these values aremerely intended to be representative.

Because the filaments are formed from a relatively rigid ceramicmaterial, the tows 16 and 18 are also rigid and do not readily deform,with the result that the filaments are prone to breakage and the tows 16and 18 are prone to damage during the fabrication of the preform 10,during which the dynamic tows 18 are interlaced with the static tows 16.Furthermore, in typical weaving and braiding machines with limitednumbers of available weaving positions, the packing of filaments withinthe tows 16 and 18 is not well controlled and can be excessive,particularly in the case of three-dimensional weaving and braiding. Inthe past, preform fabricators have resorted to using groups of tows ineach position to create larger parts, which results in larger andtypically polygonal-shaped interstitial regions 20 between adjacentdynamic tows 18. The fabricators must then try to fill each interstitialregion 20 with a round or elliptical tow 16, which cannot be efficientlydone as evident from FIG. 1. Additionally, the tows 16 and 18 aregenerally under some amount of tensioning during weaving and braidingoperations, resulting in tightening of the tows 16 and 18 that canfurther aggravate the segregation of reinforced and unreinforcedregions, and may result in higher than desired fiber volumes. From FIG.1, it can be appreciated that the tows 16 and 18 may sustain damagewhere they are forced into contact with each other during the preformfabrication process.

FIG. 2 schematically represents a fragmentary cross-sectional view of afabric preform 30 similar to the preform 10 of FIG. 1. For example, thepreform 30 depicted in FIG. 2 can have a braided or woventhree-dimensional architect, in which a set 34 of dynamic tows 38 areinterlaced with a set 32 of substantially straight static tows 36.According to a preferred aspect of the invention, a fundamentaldifference between the preforms 10 and 30 is that the static tows 36 ofthe latter are preshaped to achieve controlled filament packing and havecross-section shapes optimized to fill the interstitial regions 40between adjacent dynamic tows 38. As previously discussed, suitablematerials for the filaments within the tows 36 and 38 include siliconcarbide, silicon nitride, silicon, and/or another ceramic material, suchas the above-noted HI-NICALON®, and suitable diameters for the filamentsare about eight to about twenty micrometers, though fibers with largerand smaller diameters are also possible. Furthermore, with respect tothe dynamic tows 38, filament counts and tow diameters can be similar tothat described for FIG. 1. Potential and necessary differences betweenthe static and dynamic tows 36 and 38 will be evident from the followingdiscussion.

FIGS. 2 and 3 represent each static tow 36 as having a cross-sectionalshape that is substantially congruent to the cross-sectional shape ofthe interstitial region 40 in which the tow 36 is located, preferably sothat the shaped static tow 36 fills or nearly fills the entireinterstitial region 40 in which it is located. As a result, the shapedstatic tows 36 more efficiently fill the interstitial regions 40 than dothe round or elliptical tows 16 of FIG. 1. For example, the static tows36 preferably fill at least 80% of the cross-sectional area of eachinterstitial region 40. While diamond and triangular-shaped static tows36 are represented in FIGS. 2 and 3, other cross-sectional shapes arealso within the scope of this invention, particularly other polygonalshapes that are congruent to the cross-sectional shapes of theinterstitial regions 40 to be filled.

To obtain a predefined cross-sectional shape, the static tows 36 arepreferably prefabricated, for example, in the form of a composite rodstock or a heavily sized tow, by which a bundle of filaments areimpregnated or otherwise embedded in a temporary matrix material. If theformer, the tows 36 can be fabricated by a pultrusion process. Forexample, loose filaments are impregnated with the matrix material, entera die, and exit as a cured rod stock, all while being continuouslypulled through the die. The die preferably maintains filament alignment,controllably compresses the filaments to a desired volume fraction, andcures the matrix material in a relatively short period of time withoutdamaging the filaments. If in the form of a heavily sized tow, thestatic tows 36 can be fabricated in a manner similar to the static tows16 of FIG. 1, but using a sizing material as the temporary matrixmaterial for the filaments. In other words, each tow 36 is effectivelyimpregnated with the matrix material that surrounds the filaments withinthe tow 36.

The temporary matrix material of the tows 36 is not intended to remainas a permanent constituent of the final CMC material, but instead isremoved prior to infiltration with the permanent matrix material (or itsprecursor) desired for the CMC material. For this reason, suitabletemporary matrix materials for the static tows 36 include materials thatcan be cleanly and completely removed, such as low char-yield polymerscapable of being burned-off by heating to temperatures that can besustained by the tows 36 and 38 without damage. Most epoxies arebelieved to be acceptable, as are polyvinyl alcohol (PVA) and othermaterials that leave little or no carbon char residue when heated tosuitable burn-off temperatures, for example, about 700° C. to about 750°C.

During fabrication of the static tows 36, the amount of temporary matrixmaterial impregnated into the tows 36 can be tailored to achieve adesired volume fraction of the filaments within the tows 36 (forexample, about 20% to about 40%), corresponding to tow packing fractionor spacing of filaments within a conventional tow such as the statictows 16 of FIG. 1. The temporary matrix material for the static tows 36also has the benefit of protecting the brittle filaments within thestatic tows 36 during fabrication of the preform 30, as well as thefilaments of the dynamic tows 38 that contact the static tows 36 duringor after the preform fabrication process. During prefabrication of thestatic tows 36, the amount of matrix material present at the exteriorsurface of each tow 36 can also be tailored to promote a predeterminedcross-sectional shape for the tow 36. Following burn-off of the matrixmaterial, only the controllably-packed filaments of the static tows 36remain, with the result that the spacing between the tows 36 and 38 maybe slightly though uniformly increased. As a result, infiltration of thepreform 30 during composite densification with the ceramic matrixmaterial or its precursor is greatly enhanced, thus providing thecapability of also greatly enhancing the properties of the final CMCcomponent and reducing material scrap rates. Various infiltrationtechniques can be used for this final step, a notable example of whichis chemical vapor infiltration (CVI) to penetrate the small gaps betweenthe tows 36 and 38. Matrix infiltration by melt infiltration (MI) canalso be used, for example, by slurry cast-MI or resin transfer molding(RTM) with a resinous slurry followed by MI. Such infiltrationtechniques are well known in the art and therefore do not need furtherexplanation. FIG. 4 schematically represents the completion of thisstep, in which the preform 30 of FIG. 2 is shown as infiltrated with aceramic matrix material 48 to yield a CMC component 50.

While the invention has been described in terms of particularembodiments, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

The invention claimed is:
 1. A three-dimensional preform produced by a process comprising the steps of: producing a first set of tows in which each tow contains a plurality of substantially parallel filaments embedded within a temporary matrix, the tows of the first set comprising first tows and second tows having predetermined cross-sectional shapes that differ from each other; producing at least a second set of tows in which each tow contains a plurality of substantially parallel filaments; and then fabricating the three-dimensional preform to comprise the first and second sets of tows, in which the tows of the second set are transverse to the tows of the first set, the tows of the first set are substantially straight, the tows of the second set are interlaced with the tows of the first set to define interstitial regions that have different cross-sectional shapes and are each between adjacent tows of the second set, the tows of the first set are within the interstitial regions, and the predetermined cross-sectional shapes of the first tows of the first set are substantially congruent to a first of the different cross-sectional shapes of the interstitial regions and the predetermined cross-sectional shapes of the second tows of the first set are substantially congruent to a second of the different cross-sectional shapes of the interstitial regions that is different from the first of the different cross-sectional shapes of the interstitial regions so that the first and second tows of the first set substantially fill the interstitial regions.
 2. The three-dimensional preform according to claim 1, wherein the filaments are ceramic materials.
 3. The three-dimensional preform according to claim 1, wherein the preform is fabricated such that the first set of tows are static tows and the second set of tows are dynamic tows of the preform.
 4. The three-dimensional preform according to claim 1, wherein the predetermined cross-sectional shapes of the interstitial regions and of the tows of the first set are polygonal.
 5. The three-dimensional preform according to claim 1, wherein the preform has a woven architecture.
 6. The three-dimensional preform according to claim 1, wherein the preform has a braided architecture.
 7. The three-dimensional preform according to claim 1, wherein the tows of the first set are produced by a pultrusion process.
 8. The three-dimensional preform according to claim 1, wherein the tows of the first set are produced by a process during which the tows of the first set are impregnated with the temporary matrix.
 9. The three-dimensional preform according to claim 1, wherein the predetermined cross-sectional shapes of the first and second tows of the first set are, respectively, diamond and triangular shapes.
 10. A composite component comprising a matrix material reinforced with the three-dimensional preform produced by the process according to claim 1, the composite component being produced by a process comprising the steps of: heating the preform to remove the temporary matrix on the first set of tows so as to yield substantially uniform spacing between the first and second sets of tows; and then infiltrating the spacing between the first and second sets of tows to form the composite component and so that the preform is within the matrix material.
 11. The composite component according to claim 10 wherein, during the heating of the preform, the temporary matrix burns off without leaving any amount of residue on the tows of the first set that would reduce the mechanical properties of the composite component.
 12. The composite component according to claim 10, wherein the component is a ceramic matrix composite component of a gas turbine engine.
 13. A three-dimensional preform comprising: a first set of tows in which each tow contains a plurality of substantially parallel filaments embedded within a temporary matrix, the tows of the first set comprising first tows and second tows having predetermined cross-sectional shapes that differ from each other; at least a second set of tows in which each tow contains a plurality of substantially parallel filaments; wherein the tows of the second set are transverse to the tows of the first set, the tows of the first set are substantially straight, the tows of the second set are interlaced with the tows of the first set to define interstitial regions that have different cross-sectional shapes and are each between adjacent tows of the second set, the tows of the first set are within the interstitial regions, and the predetermined cross-sectional shapes of the first tows of the first set are substantially congruent to a first of the different cross-sectional shapes of the interstitial regions and the predetermined cross-sectional shapes of the second tows of the first set are substantially congruent to a second of the different cross-sectional shapes of the interstitial regions that is different from the first of the different cross-sectional shapes of the interstitial regions so that the first and second tows of the first set substantially fill the interstitial regions.
 14. The three-dimensional preform according to claim 13, wherein the filaments are ceramic materials.
 15. The three-dimensional preform according to claim 13, wherein the first set of tows are static tows and the second set of tows are dynamic tows of the preform.
 16. The three-dimensional preform according to claim 13, wherein the predetermined cross-sectional shapes of the interstitial regions and of the tows of the first set are polygonal.
 17. The three-dimensional preform according to claim 13, wherein the preform has an architecture chosen from the group consisting of woven architectures and braided architectures.
 18. The three-dimensional preform according to claim 13, wherein the predetermined cross-sectional shapes of the first and second tows of the first set are, respectively, diamond and triangular shapes.
 19. A gas turbine engine component comprising a matrix material reinforced with the three-dimensional preform according to claim 13, the component being produced by a process comprising the steps of: heating the preform to remove the temporary matrix on the first set of tows so as to yield substantially uniform spacing between the first and second sets of tows; and then infiltrating the spacing between the first and second sets of tows to form the component and so that the preform is within the matrix material.
 20. A gas turbine engine component comprising a ceramic matrix material reinforced with a three-dimensional preform, the three-dimensional preform comprising: a first set of tows in which each tow contains a plurality of substantially parallel filaments, the tows of the first set comprising first tows and second tows having predetermined cross-sectional shapes that differ from each other; and at least a second set of tows in which each tow contains a plurality of substantially parallel filaments; wherein the tows of the second set are transverse to the tows of the first set, the tows of the first set are substantially straight, the tows of the second set are interlaced with the tows of the first set to define interstitial regions that have different cross-sectional shapes and are each between adjacent tows of the second set, the tows of the first set are within the interstitial regions, the predetermined cross-sectional shapes of the first tows of the first set are substantially congruent to a first of the different cross-sectional shapes of the interstitial regions, and the predetermined cross-sectional shapes of the second tows of the first set are substantially congruent to a second of the different cross-sectional shapes of the interstitial regions that is different from the first of the different cross-sectional shapes of the interstitial regions so that the first and second tows of the first set substantially fill the interstitial regions, and the ceramic matrix material is within the interstitial regions between the first and second sets of tows so that the preform is within the ceramic matrix material. 